Date: Fri, 4 Mar 2016 14:38:01 -0800
From: Darren Hart <dvhart@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Jianyu Zhan <nasa4836@gmail.com>, LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>, dave@stgolabs.net,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>, dvhart@linux.intel.com,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Fengguang Wu <fengguang.wu@intel.com>, bigeasy@linutronix.de
Subject: Re: [PATCH] futex: replace bare barrier() with more lightweight
 READ_ONCE()
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On Fri, Mar 04, 2016 at 01:57:20PM -0800, Paul McKenney wrote:
> On Fri, Mar 04, 2016 at 01:05:24PM -0800, Darren Hart wrote:
> > On Fri, Mar 04, 2016 at 09:12:31AM +0800, Jianyu Zhan wrote:
> > > On Fri, Mar 4, 2016 at 1:05 AM, Darren Hart <dvhart@infradead.org> wrote:
> > > > I thought I provided a corrected comment block.... maybe I didn't. We have been
> > > > working on improving the futex documentation, so we're paying close attention to
> > > > terminology as well as grammar. This one needs a couple minor tweaks. I suggest:
> > > >
> > > > /*
> > > >  * Use READ_ONCE to forbid the compiler from reloading q->lock_ptr and
> > > >  * optimizing lock_ptr out of the logic below.
> > > >  */
> > > >
> > > > The bit about q->lock_ptr possibly changing is already covered by the large
> > > > comment block below the spin_lock(lock_ptr) call.
> > > 
> > > The large comment block is explaining the why the retry logic is required.
> > > To achieve this semantic requirement,  the READ_ONCE is needed to prevent
> > > compiler optimizing it by doing double loads.
> > > 
> > > So I think the comment above should explain this tricky part.
> > 
> > Fair point. Consider:
> > 
> > 
> > /*
> >  * q->lock_ptr can change between this read and the following spin_lock.
> >  * Use READ_ONCE to forbid the compiler from reloading q->lock_ptr and
> >  * optimizing lock_ptr out of the logic below.
> >  */
> > 
> > > 
> > > > /* Use READ_ONCE to forbid the compiler from reloading q->lock_ptr  in spin_lock()  */
> > > 
> > > And as for preventing from optimizing the lock_ptr out of the retry
> > > code block,  I have consult
> > > Paul Mckenney,  he suggests one more  READ_ONCE should be added here:
> > 
> > Let's keep this discussion together so we have a record of the
> > justification.
> > 
> > +Paul McKenney
> > 
> > Paul, my understanding was that spin_lock was a CPU memory barrier,
> > which in turn is an implicit compiler barrier (aka barrier()), of which
> > READ_ONCE is described as a weaker form. Reviewing this, I realize the
> > scope of barrier() wasn't clear to me. It seems while barrier() ensures
> > ordering, it does not offer the same guarantee regarding reloading that
> > READ_ONCE offers. So READ_ONCE is not strictly a weaker form of
> > barrier() as I had gathered from a spotty reading of
> > memory-barriers.txt, but it also offers guarantees regarding memory
> > references that barrier() does not.
> > 
> > Correct?
> 
> If q->lock_ptr is never changed except under that lock, then there is
> indeed no reason for the ACCESS_ONCE().

The only location where a q->lock_ptr is updated without that lock being held is
in queue_lock(). This is safe as the futex_q is not yet queued onto an hb until
after the lock is held (so unqueue_me() cannot race with queue_lock()).

> So, is q->lock_ptr ever changed while the lock is -not- held?  If so,
> I suggest that you put an ACCESS_ONCE() there.

It is not.

-- 
Darren Hart
Intel Open Source Technology Center